In recent years, the introduction of distributed power sources, typified by a solar power generation, into the power distribution system has been actively promoted, but as a result, increasing voltage variation of the power distribution system is becoming a problem. It is effective to apply a reactive power compensator for improving the stability of such a power distribution system, and in particular, application of a cascade-connection type SVC (Static Var Compensator) is proposed.
In Japanese Patent Application JP2007-280358A, a technique of a cascade-connection type SVC is disclosed, wherein single-phase power converters, having N units per phase, are serially connected at the AC side and configured to be capable of outputting multi-level voltages.
When the single-phase power converters (hereinafter, referred to “cells” as appropriate) are controlled by a PWM (Pulse Width Modulation) method, and if a phase of a triangular carrier wave to be supplied to each of the cells is shifted properly, it is possible for output power waveforms of the SVCs to approximate sine waves at multi-levels, thereby suppressing harmonic components.
In Japanese Patent Application JP2002-345252A, an operation method is disclosed, wherein, in order to control a plurality of cells by a PWM method, by constituting a distributed control system consisting of: a central control unit installed in a position at a distance from the cells; and cell control units installed in the vicinity of each of the cells, and by including a PWM synchronization signal, as well as a voltage command value and a PWM pattern command, in a signal to be transmitted from the master to the slaves, a PWM generator is reset every time the synchronization signal is outputted.